1. Field of the Invention
The present invention relates to a directional coupler used in the microwave or millimeter-wave band, and to a high-frequency circuit device including the same.
2. Description of the Related Art
In the art, line couplers such as hybrid-ring couplers are used as microwave directional couplers having lines on a substrate. Characteristics of such line couplers, such as the power distribution ratio, are determined by appropriately designing the line lengths and the characteristic impedances of lines that connect four ports.
In line couplers however, in a high-frequency region, namely the millimeter-wave band of a propagating signal, the line lengths of the lines that connect the ports are short while the line widths are relatively wide. Thus, it is difficult to form a line pattern on the substrate.
One directional coupler used in the millimeter-wave band or the like that overcomes the foregoing problem is disclosed in Marek E. Bialkowski, Senior Member, IEEE, and Shaun T. Jellett, Member, IEEE, “Analysis and Design of a Circular Disc 3 dB Coupler,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 8, AUGUST 1994. FIG. 17 shows the structure of the directional coupler disclosed in this publication. As shown in FIG. 17, the directional coupler includes a round conductor C, lines L1, L2, L3, and L4 radially extending in four directions from the round conductor C, and open-end stubs S1 and S2. The stub S1 projects from the round conductor C between the lines L1 and L4, and the open-end stub S2 projects from the round conductor C between the lines L2 and L3.
The directional coupler shown in FIG. 17 has a symmetry axis that extends through the two stubs S1 and S2, and another symmetry axis orthogonal to this axis. Thus, a plurality of resonant modes occur in the round conductor C. Without the stubs S1 and S2, the resonant modes are degenerate, whereas, with the stubs S1 and S2, degeneracy of the modes is broken, exhibiting directional coupler characteristics.
However, the directional coupler shown in FIG. 17 experiences a problem in that it has many design parameters including, the line width of the lines L1 to L4, the radius of the round conductor C and the shape and size of the stubs S1 and S2, i.e., the stub length and the stub width, resulting in a high level of difficulty in its design. Moreover, changes in the directional coupler characteristics are highly susceptible to errors in the pattern accuracy of the stubs S1 and S2, the round conductor C, and the lines L1 to L4, that is, high pattern accuracy is required for the desired electrical characteristics. It is therefore difficult to form a conductor pattern on a dielectric substrate using, for example, a thick film printing technique.